As the widespread use of the Internet and mobile phones has increased communication capacities in recent years, backbone optical communication systems have been required to have larger capacities, and research and development have been carried out for optical transmitter-receivers having a communication capacity of 40 Gbit/s, 100 Gbit/s or higher for a single wavelength.
However, if a transmission capacity per single wavelength is increased, the quality of transmitted signals deteriorates greatly due to a lowered OSNR (Optical Signal to Noise Ratio), waveform distortions caused by wavelength dispersion on transmission paths, polarization mode dispersion, nonlinear effects, and the like.
Accordingly, digital coherent receiving schemes resistant to poor OSNR and also resistant to waveform distortions on transmission paths have been gathering attention as schemes for optical communication systems yielding 40 Gbit/s or higher.
According to conventional receiving schemes, ON and OFF settings based on light intensities are assigned to binary signals to be used for direct detection (OOK: On-OFF Keying). By contrast, according to digital coherent receiving schemes, light intensity and phase information are extracted using a coherent receiving system, and the extracted light intensity and phase information are quantized using an analog/digital converter (ADC), and thereby demodulation is performed by a digital signal processing circuit.
Digital coherent receiving schemes are capable of improving resistance to poor OSNR by using coherent receiving schemes, and are capable of compensating for waveform distortions by using a digital signal processing circuit, and accordingly are capable of suppressing deteriorations in the quality of transmitted signals even when a communication capacity for one wavelength is large. Also, wavelength distortions may be compensated for by a digital signal processing circuit, which enables relatively flexible responses to transmission route modifications caused by network configuration modifications.
Further, digital coherent receiving schemes can be combined with modulation schemes capable of transmitting multi-bit information for one symbol so as to construct transmission systems yielding high-frequency efficiencies. As modulation schemes of this type, multivalued modulation schemes such as QPSK (quadri-phase shift keying), 8PSK, 16QAM, and 256QAM that multiply phase information and intensity information, a polarized multiplexing scheme that multiplexes different information onto orthogonal polarized waves, a multi-carrier multiplexing scheme that multiplexes different information onto a plurality of frequencies that have been multiplexed highly densely within one wavelength grid (subcarriers), etc., are known. As typical examples of multi-carrier multiplexing schemes, there are Frequency Division Multiplexing (FDM) and Orthogonal Frequency Division Multiplexing (OFDM). As a method of realizing OFDM, there is a method, as described in non-Patent Document 1, in which a plurality of phase synchronized subcarrier signals (multi-frequency light sources) are modulated.
There are also active discussions on techniques for flexibly modifying network configurations so as to improve the use efficiency of network capacities, and digital coherent receiving schemes capable of performing flexible compensation for linear waveform distortions are gathering attention.
However, in order to respond to requests from users of a network, not only the traffic volume of the network, but also a place to which data is to be transmitted (transmission distance) has also to be considered. Thus, flexible responses to such requests are also needed. Also, conditions of systems that transmit signals (such as an optical SNR, a nonlinear effect, wavelength arrangements, and the like) vary depending upon networks. Flexible responses to such a variety of conditions are also desired.
In order to respond to requests such as those described above, it is desirable to be able to flexibly vary operations of transmission units such as modulation scheme, modulation rate (Baud Rate), etc. in response to system requirements (capacity, bandwidth, and distance) and conditions (optical SNR and non-linear effects).
FIGS. 1 and 2 explain conventional techniques.
FIG. 1(a) illustrates a conventional example of a frequency arrangement using a multi-frequency light source. In FIG. 1(a), OOK signals of 10 Gb/s, DPSK signals of 40 Gb/s, and FDM-QPSK signals of 100 Gb/s are disposed, and these signals are not arranged in a right order in view of frequencies, causing interference between adjacent signal bandwidths and thus signal deterioration. Accordingly, as illustrated in FIG. 1(b), signal bandwidths are assigned in such a manner that signals modulated in similar modulation schemes are adjacent to each other to the extent possible. In FIG. 1(b), there are signals of 1.12 Tb/s, signals of 440 Gb/s, and signals of 100 Gb/s, and they are arranged as groups in three signal bandwidths, respectively. It is desirable to transmit signals using the fewest possible number of signal bandwidths that contain signals modulated by a plurality of different modulation schemes by varying flexible modulation schemes of individual frequencies.
FIG. 2 illustrates a configuration example of a conventional transmitter that uses a multi-frequency light source.
A single-frequency light is output from a laser 10 as depicted by (a). The single-frequency light output from the laser 10 is converted, by a multi-carrier generator 11, into light having a plurality of frequency carriers as depicted by (b). A demultiplexer 12 demultiplexes respective frequencies generated by the multi-carrier generator 11, and outputs those frequencies to a modulator 13. In this example, a polarization-diversity I/Q modulator array 13 is represented as an example of a modulator. This modulator 13 performs modulation using an I signal and a Q signal, and superposes different signals onto orthogonal polarized waves to output them. Optical signals from the modulator 13 are multiplexed by a coupler 14 to be output. The frequency spectrum of optical signals being output has widths around the respective frequencies due to the modulation as depicted by (c).
When an OFDM signal is applied using a multi-frequency light source described in non-Patent Document 1, it is possible to vary the modulation scheme (multi-valued degree) of each subcarrier and the number of subcarriers so as to respond to the traffic volume of networks. However, a single OFDM signal has a fixed frequency difference between the carriers and a fixed Baud Rate, which prevents a flexible and sufficient response to requests that are made for satisfying system requirements.
For a transmitter using a multi-frequency light source, it is necessary to be able to vary the multi-valued degree of each subcarrier and the number of subcarriers, to vary the frequency difference between respective subcarriers and the Baud Rate of each subcarrier in addition to responding to increases or decreases in the traffic volume, and to apply a different Baud Rate to subcarriers so that changes in transmission distances can be responded.
As conventional techniques, there is a technique by which single-carrier-signal generation units are made to respond to a plurality of wavelength bandwidths so as to increase the number of single-carrier-signal optical bandwidths, a technique by which one of the phases of two LDs are controlled in response to detected phase signals in order to increase the number of channels, a technique by which light other than an optical pulse is suppressed by using an optical gate in order to improve the optical SNR, and a technique by which two pulse light generation circuits having the same frequency for repeating pulse light but different oscillation frequencies are used in order to generate a multi-wavelength light having a broad bandwidth and equal frequency intervals utilizing the nonlinear optical effect.    Patent Document 1: Japanese Laid-open Patent Publication No. 2010-124320    Patent Document 2: Japanese Laid-open Patent Publication No. 2009-244304    Patent Document 3: Japanese Laid-open Patent Publication No. 2002-062514    Patent Document 4: Japanese Laid-open Patent Publication No. 2001-249367    non-Patent Document 1: “Terabit Superchannels for High Spectral Efficiency Transmission”, S. Chandrasekhar, et. al., Tu.3.C.5, ECOC2010.